Adjust test-http-content-length and docs to consider countdown

doc/guides/writing-tests.md

@@ -133,11 +133,15 @@ platforms.
 
 ### The *common* API
 
-Make use of the helpers from the `common` module as much as possible.
+Make use of the helpers from the `common` module as much as possible. Please refer
+to the [common file documentation](https://github.com/nodejs/node/tree/master/test/common)
+for the full details of the helpers.
 
-One interesting case is `common.mustCall`. The use of `common.mustCall` may
-avoid the use of extra variables and the corresponding assertions. Let's explain
-this with a real test from the test suite.
+#### common.mustCall
+
+One interesting case is `common.mustCall`. The use of `common.mustCall` may avoid
+the use of extra variables and the corresponding assertions. Let's explain this
+with a real test from the test suite.
 
 ```javascript
 'use strict';
@@ -189,6 +193,23 @@ const server = http.createServer(common.mustCall(function(req, res) {
 });
 
 ```
+#### Countdown Module
+
+The common [Countdown module](https://github.com/nodejs/node/tree/master/test/common#countdown-module) provides a simple countdown mechanism for tests that
+require a particular action to be taken after a given number of completed tasks
+(for instance, shutting down an HTTP server after a specific number of requests).
+
+```javascript
+const Countdown = require('../common/countdown');
+
+const countdown = new Countdown(2, function() {
+  console.log('.');
+});
+
+countdown.dec();
+countdown.dec(); // The countdown callback will be invoked now.
+```
+
 
 ### Flags
 

test/parallel/test-http-content-length.js

@@ -2,6 +2,7 @@
 require('../common');
 const assert = require('assert');
 const http = require('http');
+const Countdown = require('../common/countdown');
 
 const expectedHeadersMultipleWrites = {
   'connection': 'close',
@@ -18,8 +19,8 @@ const expectedHeadersEndNoData = {
   'content-length': '0',
 };
 
-let receivedRequests = 0;
-const totalRequests = 3;
+
+const countdown = new Countdown(3, () => server.close());
 
 const server = http.createServer(function(req, res) {
   res.removeHeader('Date');
@@ -42,8 +43,7 @@ const server = http.createServer(function(req, res) {
       throw new Error('Unreachable');
   }
 
-  receivedRequests++;
-  if (totalRequests === receivedRequests) server.close();
+  countdown.dec();
 });
 
 server.listen(0, function() {